The prior art has used various audible signals, ringers, buzzers, etc., as well as visual signals such as light bulbs, to alert a party of the arrival of an incoming call to the party's telephone station set. The audible alerting tones that are produced at the telephone station set to signal an incoming call are typically constant in volume regardless of the busy/idle state of the telephone. Therefore, if the called party is already off-hook and engaged in an existing call, the audible alerting signal of a second incoming call may be very disturbing and irritating, and especially so if the existing call is on a speakerphone. Because speakerphones generally permit only one-way communications, i.e., switching between transmit and receive depending on which party is presently speaking, or is the loudest if both started speaking simultaneously, an incoming alerting signal may not only disturb and annoy the two parties but may also disrupt communications between the two parties.
The prior art also has taught the use of the speakerphone of a telephone station set to generate audible alerting signals. When the set is on-hook, it responds to receipt of notification of an incoming call (e.g., a ringing signal on the telephone line) by connecting an alerting signal generator to the loudspeaker of the speakerphone and driving the loudspeaker with output of the alerting signal generator to produce the audible alerting tone. When the set is off-hook, however, the alerting signal generator is not used to produce an audible alerting tone. In order to cause the speakerphone loudspeaker to produce an audible alerting tone in this instance, it is necessary for the set to receive an alerting tone directly over the telephone line, like a voice signal, which alerting tone is then applied by the set to the speakerphone loudspeaker just like a received voice signal, and is thusly made audible by the loudspeaker.
The disadvantages of this prior art approach are evident: it requires two separate mechanisms for audible alerting tone generation, one for use when the set is on-hook and a different one for use when the set is off-hook. Plus, the latter mechanism requires that the entity to which the set is connected by the telephone line (e.g., a telephone switching system such as a central office switch or a private branch exchange) have the capability of generating alerting signals in voice signal form.
Merely allowing one of the prior art speakerphone alerting mechanisms to be used at all times, regardless of whether the set is on-hook or off-hook, does avoid some disadvantages, only to replace them with new ones. On one hand, use of the prior art off-hook mechanism at all times requires the set to be able to respond to voice signal-type alerting signals while it is on-hook-no easy or inexpensive task. On the other hand, use of the prior art on-hook mechanism at all times creates the problems set forth at the beginning of this discussion. Moreover, these problems are especially acute for telephone station sets that use the speakerphone to generate the audible alerting signal. In the speakerphone, the loudspeaker and the microphone are located in close proximity to each other, very much closer than is typically the case between a conventional telephone station set ringer and a microphone of the station set when the set is off-hook. Consequently, the speakerphone loudspeaker-generated alerting signals more readily and more strongly affect the speakerphone microphone and cause more interference, annoyance, and disruption thereby, than would typically be true of non-speakerphone generated alerting signals.